The following is a good article on scouting for insects and mites in the greenhouse from Rutgers University.
Scouting Options and Methods
There are essentially three options available when scouting your greenhouse crops for insect/mite pests. 1- No scouting performed with pesticides being applied on a calendar timetable. 2- Simply scouting for pest occurrence with pesticides applied when presence is observed. 3- Scouting crop and making pesticide application decisions based on pest counts and action thresholds. The third option is part of an integrated pest management (IPM) approach that has been promoted throughout the green industry the past few decades. Greenhouse pest populations are measured by trapping or direct plant inspection and both involve determining pest numbers.
Counting pests and using action thresholds requires time and knowledge, but results in less pesticide use and can improve plant quality. It is important to remember that trapping (e.g. yellow or blue sticky cards) improves the efficiency when scouting your greenhouse, but does not replace the actual inspection of individual crop plants. This is particularly the case when scouting for aphids and mites. Benefits of Counting Pests The scouting and counting of insects/mites helps to detect when they are first present. Therefore treatments are made before large populations build up, but not before it becomes necessary.
Tracking pest numbers over a period of time allows for the use of action thresholds, or when pest density levels threaten crop salability and economic loss. When pest densities and damage are low, it is not efficient to spend 95% of your time controlling the last 5% of the pest.
The use of biological controls (e.g., beneficial insect/mite augmentation) is most effective when pest numbers are low, and scouting helps to know their density and location so that natural enemy release can be targeted. When using biological controls, pest count estimates are required in order to determine how many beneficials to release per target management area. Finally, instead of guessing, scouting and estimating pest counts makes it possible to evaluate the effectiveness of pest control interventions after they are applied.
Using Sticky Cards to Trap Adults
Generally one (1) sticky card is placed within each 1,000 sq. ft. area and also near greenhouse vent and door openings. Ideally the card should be placed at the level of the crop canopy or slightly below to effectively trap many of the major adult pests found in the greenhouse. Each of the sticky cards should be examined at least once per week. Using stakes and double clothespins to support the traps is effective. Also, be certain to number and date each trap card to a specific location. When pest counts are low it is acceptable to re-use the trap card for additional weeks.
Yellow sticky card
Information from "Pest Counts and Action Thresholds in the Greenhouse" by Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University in the August 20, 2009 edition of the Plant & Pest Advisory, Landscape, Nursery & Turf Edition; A Rutgers Cooperative Extension Publication